The present invention relates generally to an optical inspection system for determining the characteristics of a moving web. More specifically, the present invention relates to an optical inspection system for determining the characteristics of a moving web of cigarette paper containing bands.
Commonly assigned U.S. Pat. Nos. 5,417,228 and 5,474,095 disclose cigarette papers comprising a base web and banded regions of add-on material. As illustrated in FIG. 1, an exemplary cigarette 7 might contain two bands 5 of material formed by depositing a layer of cellulosic pulp on base cigarette paper 3. Cellulon, microcrystalline cellulose, flax or wood pulp, or amylopectin are some of the various preferred substances which have been used to form the bands.
Commonly assigned U.S. Pat. No. 5,534,114 discloses that the above described bands can be formed by modifying a conventional Fourdrinier paper making machine to deposit additional layers of cellulose at some stage in the production of the cigarette base paper 3. To streamline process, the bands are preferably applied while the paper is moving at high speeds, such as 500 feet per minute. At these high speeds, breakdowns and other factors (such as clogged band applicators), can result in the production of a base web having misplaced bands.
For example, as illustrated in FIG. 2, common anomalies arise when the width of a band 1 deviates from a desired width 12, or the band becomes skewed so that it is no longer orthogonal with respect to the edge of the paper (as is the case with band 1). Other anomalies arise when the separation 2 between two bands deviates from a desired separation width 10 (also called "band spacing" herein). Moreover, an irregular band applicator may produce a band with gaps or a band having a contrast which is either too high (e.g. as in band 9) or too low.
The prior art includes web inspection devices for use in the manufacture of fabrics, film, paper and like material. Some of these devices include a light source for projecting electromagnetic radiation on a moving web of material. The light impinges on the surface of the moving web, where it is reflected and received at a detector device. Any anomalies in the moving web can be detected by investigating the nature of the reflected electromagnetic radiation. For instance, a tear, pinhole or blemish in the web will manifest itself in a spike in the signal level from the detector (which is attributed to an increase or decrease in reflected radiation). This spike can be viewed by connecting the detector output to an oscilloscope, as exemplified by U.S. Pat. No. 5,426,509 to Peplinkski.
While useful, these devices are ill-suited to the task of detecting the integrity of bands on cigarette paper. Bands formed on cigarette paper often have reflective properties similar to the cigarette paper itself. Often, for instance, the bands are formed of white colored material which is difficult to distinguish from the white colored cigarette paper. Moreover, the basis weight of the cigarette paper may vary along the direction of travel of the paper on the paper making machine (due to the difficulty in maintaining a constant pulp application rate). The variance in basis weight of the paper influences its reflective properties, thereby obfuscating the differences between banded and non-banded regions, which are subtle enough to begin with. The prior art devices do not have the ability to interpret a reflection from a web of this nature. As mentioned, these devices are configured to examine a web surface for tears, pinholes and blemishes which manifest themselves in dramatic spikes in the video camera signal.
Also, whether a band width is too long, too short, or separated from its neighboring band by more or less than a desired distance can not be determined by simply observing the properties of a single point on a moving web. Rather, the properties of a band should be gauged by determining the spatial relationship between different elements on the web.
Pattern recognition techniques are one way of determining the spatial relationship between different features on a printed web of material. In a common technique, a camera forms a digital image of a portion of a web of material and information printed thereon. The digital image is then compared with a pre-stored template representing an error-free web portion. Discrepancies between the template and the image represent an irregular web. These techniques offer accuracy, but unfortunately entail a great deal of data processing. These techniques are therefore ill-suited to detecting the properties of bands on a web which may be moving at speeds of greater than or equal to 500 feet per minute.
Accordingly, it is an exemplary objective of the present invention to provide an inspection system for accurately detecting the properties of bands contained on a moving web of cigarette paper without delaying other stages in the manufacture of the cigarette paper.